In recent years, a threaded shaft has been used as a mehanism for intermittently moving a carriage on which magnetic head is carried, radially of a magnetic disk. A recently developed device employing the threaded shaft for moving such a carriage is shown in FIG. 4, where the threaded shaft, indicated by numeral 1, is connected to a stepping motor (not shown). The shaft 1 has a helical groove 2 formed on its outer surface. A mating element 4 mounted to a carriage 3 engages the groove 2. The front end 4a of the mating element 4 takes the form of a somewhat rounded quadrangular pyramid. The mating element 4 is mounted to the central portion 5b of a leaf spring 5 of a U-shaped cross section. Both ends 5a of the spring 5 are rigidly fixed to both side walls of the carriage 3 with threaded members 6 and 7. The mating element 4 is pressed against the groove 2 by the resilient force of the spring 5. Magnetic head 8 is carried on the upper surface of the carriage 3. Two guide shafts 9 and 10 fixedly secured to a chassis (not shown) allow the carriage 3 to be moved parallel to the shaft 1.
In the conventional device as described above, when the shaft 1 is rotated in a forward or reverse direction by the stepping motor, the carriage 3 moves a distance corresponding to the angle through which the shaft 1 rotates, along the guide shafts 9 and 10 rearward or forward because the mating element 4 engages the groove 2, in the manner described below.
As can be seen from FIG. 4, the helical groove 2 is formed so as to run in a direction perpendicular to the axis of the shaft 1. The groove 2 has non-advancing portions 2a, which have a lead angle of 0.degree. and are axially spaced apart from each other. The groove 2 further has advancing portions 2b formed between the non advancing portions 2a, the advancing portions 2b being inclined to the axis of the shaft 1. One rotation of the shaft 1 corresponds to eight non-advancing portions 2a and eight advancing portions 2b. The relation of the distance traveled by the carriage 3 to the angle through which the shaft 1 rotates is shown in FIG. 5. The device is so designed that the stepping motor stops when the angle reaches 0.degree., 45.degree., 90.degree., 135.degree., 180.degree., 225.degree., 270.degree., and 315.degree. and that the front end 4a of the mating element 4 is located at the center of any non-advancing portion 2a of the helical groove 2 whenever the motor stops as described above. Thus, even if rotary motion is transmitted from the motor while involving some error, or if the motor undergoes damped oscillation after quick stoppage of the motor, it is assured that the carriage 3 stops in position as long as the front end 4a of the mating element 4 engages any non-advancing portion 2a of the groove 2.
As mentioned above, in the prior art techniques, the mating element 4 is held by the leaf spring 5 to bring the element 4 into engagement with the helical groove 2 without rattling. A gap 11 is formed between the central portion 5b of the spring 5 of a U-shaped cross section and the body of the carriage 3. Therefore, when the shaft 1 is rotated fast or stopped quickly or a large external shock is applied to the shaft 1, the leaf spring 5 may be bent excessively as shown in FIG. 6. Then, the mating element 4 may disengage from one of the roots of the groove 2 and ride on its neighboring crest or even ride over this crest and engage its neighboring root. This will prevent the magnetic head 8 from being correctly moved in a stepwise manner. As a result, correct tracking on the magnetic disk is rendered difficult. Further, when an external shock is applied, the mating element 4 may be accelerated while kept away from the groove 2. Then, the carriage 3 will collide on the chassis of the disk drive or other component, so that the carriage 3, the magnetic head 8, and other components will become damaged.